Rapid increase in consumer interest in a variety of technologies such as portable power tools calculators, computers, cordless telephones, garden tools, portable televisions, radios, tape recorders, as well as back-up power sources in computer technology, memory devices, security systems, to name a few, has increased the demand for reliable, ligh-weight, high-energy power sources. Recent developments in high energy density cells have focused attention on anodes of alkali metal, non-aqueous electrolytes and cathode-active materials of nonstoichiometric compounds. Alkali metals, particularly lithium, are use as anode-active materials because they are highly electro-negative, thereby providing high energies per weight unit. Self-discharge of cells employing alkali metal as the anode-active material is minimized by employing non-aqueous solvents which are not reducible by the highly reactive anode mateials, thus enabling an exceptionally long shelf life.
Cathode active materials generally include transition metal chalcogenides such as NbSe.sub.2, NbSe.sub.3 NbS.sub.3, MoS.sub.2, MoS.sub.3, TiS.sub.2, TiS.sub.3, TaSe.sub.3, TaS.sub.2, V.sub.6 O.sub.13, CoO.sub.2 and MoO.sub.2. These materials, especially niobium selenide, niobium triselenide, and niobium trisulfide, which are described in U.S. Pat. No. 3,864,167 issued to J. Broadhead et al. on Feb 4, 1975, have excellent capacities, good recycling properties and are very compatible with the alkali metals, especially lithium. The chalcogenide compounds are electronic conductor; however, their conductivity is not nearly as great as of metallic conductors. Therefore, the conductivity of the cathode structure is typically improved by admixing carbon black or particulate metal with the cathode-active material and/or supporting the cathode-active material on a metal grid or screen to improve current collection. Such metals may be selected from magnesium, aluminum titanium, zinc, lead, iron, nickel, copper, and alloys thereof.
U.S. Pat. No. 4.091,191 issued to Lewis H. Gaines on May 23, 1978 suggests the use of aluminum for incorporation in a particulate form (e.g. powder or fibers) into TiS.sub.2 active material and for use as a screen or perforated sheet onto which a mixture of the cathode-active material and the particulate metal is molded or pressed. The particulate metal is added in amounts of up to 50 wt.%, preferably between 5 and 20 wt.% to the active material (TiS.sub.2). Additionally, from 1 to 20 wt.% preferably from 2 to 10 wt.%, of a binder is admixed with the particulate metal and active material and the mixture is molded at pressure of up to 165 MPa (24000 psi) into a desired shape, e.g. about a perforated aluminum plate.
The disadvantages of this procedure lie in larger amounts of the particulate metal (powder or fiber) which, while adding strength and conductivity to the cathode structure, reduce the amount of cathode active material being included in a certain volume of a cathode electrode; addition of the binder further adds to the bulk of the molded material. Furthermore, expanded metal screens or grids, e.g. of nickel, cooper or thier alloys (commercially available from Exmet Corporation), rather than perforated sheet or screen of aluminum, have been typically used recently in the production of rechargeable non-aqueous alkali cells, primarily due to commercial availability and relative strength of the screen or grid made of such expanded metals. However, fibers of cathode-active material (e.g. niobium triselenide) on both sides of strands of a screen or grid of expanded metal such as nickel (e.g. 3Ni5.125) are more compressed relative to the fibers in the open areas. The compressed area exert more pressure on the separator during cycling enabling lithium to plate through the pores of the separator. The screen or grid also focuses the current at the intersection of the metal strands resulting in a nonuniform current distribution, thus increasing chances of forming an internal short. (Internal shorts are electric paths between the cathode (e.g. NbSe.sub.3 ) and the alkali metal anode (e.g. Li) that develop through a separator during the charging phase.) The cells with these internal shorts manifest charge capacities which are larger than their discharge capacities. Their capacity will also fade faster with each cycle. These problems are not unique for the expanded metal grids or screens, but may occur also in cells with cathodes in which the current collector is in the form of a flat screen or perforated sheet.
Thus, a cell with a cathode which not only ameliorates the above shortcomings, but also is adaptable for mass-production use, is highly desirable.